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Metal mixed oxide powder containing noble metal

The invention concerns a metal mixed oxide powder doped with a noble metal, its production and use.

Multi-metallic powders are known from US 5984997. The multi-metallic powders comprise at least two elements selected from the s, p, d and/or f group of the periodic table. The powders can be in aggregated or agglomerated form and display an average particle size of less than 5 μm. The elements also include catalytically active noble metals, such as gold, platinum or palladium. They are produced by burning an emulsion containing the precursor elements of the selected compound and a flammable liquid. There are many possibilities here, such as e.g. combustion in a laminar or turbulent flame, a diffusion flame, an additionally doped or undoped flame. The large number of possibilities means that many substance parameters can be adjusted in this way.

However, US 594997 does not disclose the form of the components in the powder, nor does it disclose the measures to be taken to influence the distribution of the components in the powder.

For use as a catalyst in particular, however, it is extremely important that the particles display catalytically active centres in accessible sites. In flame processes, which generally lead to particles with no significant pore structure, the catalytically active element must preferably sit on the surface of the particles.

Catalysts are generally obtained by applying a catalytically active layer from a solution with subsequent calcination. The disadvantage of catalysts produced in
this way is often the inadequate adhesion of the catalytic layer that is applied.

The object of the invention is to provide a noble metal-containing catalyst that displays as high as possible a number of accessible active noble metal centres, which are permanently bonded to a support, which can likewise be catalytically active.

The invention provides an aggregated, doped metal mixed oxide powder, which is characterised in that - the BET surface area is 30 to 150 m2/g, - the doping component is at least one noble metal,

- wherein the content of noble metal is 0.1 to 20 wt.%,' relative to the total amount of powder, and - wherein the ratio of noble metal on the surface to the total amount of noble metal is at least 0.1.

The ratio of the amount of noble metal on the surface of the aggregates to the total amount of noble metal can preferably be 0.1 to 0.3. The ratio can be determined, for example, by combined analysis of high-resolution TEM images that are representative of the sample, XPS/ESCA to determine the noble metal surface coverage and X-ray fluorescence analysis to determine the total content of noble metal . The content of noble metal covers a wide range from 0.1 to 20 wt.%. This means that the content of catalytically active noble metal can be specially adjusted to a particular subsequent use. The content is conventionally between 5 and 15 wt.%.

The noble metal on the surface of the powder according to the invention can display an average diameter of 2 to 10 nm. Particularly good catalytic activity can be expected with diameters of this order of magnitude.

The powder according to the invention can preferably contain Pt, Pd, Rh, Ru, Au or Ag as noble metal. Platinum or gold are particularly preferred. The powder according to the invention can preferably have a BET surface area of 30 to 150 m2/g and particularly preferably a BET surface area of 50 to 70 m2/g. The BET surface area is an important feature of catalytic activity and in the powder according to the invention it can be adjusted to the intended catalytic reaction. The metal mixed oxide powders within the meaning of the invention are ones which can be produced by oxidation from inorganic or organic precursors. The metal mixed oxide powders can preferably display the elements Na, K, Mg, Ca, Y, Ce, Ti, Zr, V, Nb, Mo, W, Mn, Fe, Co, Ni, Ag, Zn, Al, In, Si, Sn, Sb or Bi . Within an aggregate the metal oxides can be positioned alongside one another or can be in the form of true metal mixed oxides . Aggregates are understood to be three-dimensional structures of intergrown primary particles. Primary particles are the particles first formed in a flame during the oxidation reaction. The metal mixed oxide powders can preferably be binary oxides, in other words oxides displaying two metals. Of these, indium-tin oxide is particularly preferred. The content in wt.% of indium oxide, calculated as ln203, can preferably be 70 to 95, that of tin oxide, calculated as Sn02, 1 to 10 wt.%, relative in each case to the powder according to the invention. A powder having indium-tin oxide as the metal oxide component can preferably display platinum or gold as the noble metal component. Such a powder whose X-ray diffraction diagram shows the diffraction pattern of cubic indium oxide and an indium-platinum alloy or indium-gold alloy is particularly preferred.

The invention also provides a process for the production of the powder according to the invention wherein a solution or dispersion containing compounds with the metal components of the oxides is atomised according to the subsequently desired ratio of oxides, in a first zone of a reactor, optionally with admixture of an inert gas stream, the atomised solution is pyrolysed in a flame consisting of hydrogen and air or oxygen-enriched air, in an area comprising the last third of the flame, the noble metal component in the form of a solution or dispersion of a noble metal compound is metered into the pyrolysis mixture at one or more points, in an amount corresponding to the subsequently desired amount of noble metal and immediately afterwards reducing gases are added in a second zone, the reducing gases being added in an amount such that a reducing atmosphere is established overall in this second zone, thereafter in a third zone, in which a reducing atmosphere likewise prevails, the solid obtained is separated off.

The use of solutions is preferred within the meaning of the invention. Although dispersions can also be used, it has been found that the process allows the reaction to be controlled more easily if solutions are used. The liquid phase of the solution or dispersion can contain water, organic solvents or mixtures thereof. Water is preferably used, in order to prevent organic constituents in the powder . Suitable starting substances for the metal mixed oxides can be inorganic or organic compounds, which can be converted into the oxides under the reaction conditions.

As with the solvents, inorganic compounds are preferred here too for the same reasons . With regard to the properties of the powders according to the invention and for cost reasons, halogens can advantageously be used, chlorides being particularly advantageous . There is a limit on the number of noble metal compounds. Here too it is in principle the case that purely inorganic compounds can preferably be used. Nevertheless, good results can also be obtained with organic noble metal compounds . The following compounds that can be used are cited by way of example: PtCl2, PtCl , H [PtCl5], PdCl2, Pd acetate, Pd(N03)2, RhCl3, Rh(N03)3, RuCl3, IrCl3, H[AuCl4], AgN03. Forming gas, carbon monoxide, hydrogen, ammonia or mixtures of these gases can be used as reducing gases . The use of reducing gases is substantially in order to obtain powders having high catalytic activity. It has proved to be advantageous if the residence time is between 0.8 and 1.5 seconds in the first zone and is between 15 seconds and 15 minutes in total in the second and third zone. Figure 1 shows a schematic set-up for performing the process according to the invention. I, II and III indicate the three reaction zones. Zone la describes the' area in which the noble metal component is added. In addition: 1 = atomised solution or dispersion of the metal compounds; 2 = oxygen-containing gas, preferably air; 3 = combustion gas, preferably hydrogen; 4 = solution or dispersion containing noble metal compound; 5 = reducing gas; 6 = waste gas; 7 = powder according to the invention.

In order to be able to obtain the powder according to the invention, it is essential that the solution or dispersion of the noble metal compound be fed into
zone la. This leads to powders according to the invention wherein the noble metal component is permanently bonded to the mixed metal oxide. This can be verified by means of X-ray diffraction diagrams and XPS spectra (presence of indium-platinum bonds in XRD or strong interactions with the mixed metal oxide matrix in the XPS spectra) .

It is also essential to provide a reducing atmosphere immediately after adding the noble metal compound, in order to prevent as far as possible an undesirable oxidation of the noble metal.

Examples :

Example 1 : An aqueous solution containing 88.9 g/1 of indium(III) chloride and 8.4 g/1 of tin(IV) chloride is atomised by means of compressed air and a nozzle (diameter 0.8 mm) into the reaction tube at a delivery rate of 1500 ml/h. An oxyhydrogen flame consisting of 5 m3/h of hydrogen and 15 m3/h of air is burning there. A one-percent, aqueous H [PtCl6] solution is fed into the last third of this flame. Forming gas (5 Nm3/h) is introduced immediately after the addition of the solution.

The reaction mixture passes through a residence time section of 2 m in length in 14 seconds. The solid is then separated from the gaseous substances using a filter and treated for a period of 15 minutes at a temperature of 250°C with a continuous supply of forming gas.

Examples 2 to 4 are performed in the same way as Example 1. The feed materials and amounts used are reproduced in Table 1, the analytical data for the powders in Table 2. Figure 2 shows the X-ray diffraction diagram for the powder according to the invention from Example 3.